Section 6: Paving Operations

6.1 Introduction

Two types of paving operations are used for the construction
of rigid pavement. One is fixed-form paving, and the other is slip-form
paving. Both methods have some common operations as further delineated
here. First, both methods require accurate survey controls that
are used to establish the proper alignment and grade of the concrete
pavement. Second, both methods require proper curing of the concrete
in order to facilitate producing a durable long-lasting pavement.
Third, any required joint sawing must be accomplished in a timely
manner to prevent random cracking of the pavement.

6.2 Fixed-form Paving

Although not used very often in Texas, paving machines that
ride on forms are still in use. Paving machines that use steel wheels
to ride on paving forms are very heavy machines, almost comparable
to a slip-form paving machine without the drive tracks. To prevent
sagging under the weight of this machine, the forms need to be uniformly
supported on a very firm base.

Smaller paving machines, such as the Clary, have three long
steel roller tubes that extend across forms in both directions.
The rollers propel and screed the concrete to the level of the forms.
This type of paving machine is still seen on small and irregular
placements, such as ramps and turnarounds. It is limited to smaller
productivity and narrower placements.

Another type of paving machine that has actually been used
in recent years on main lane urban freeways is the Bidwell, a machine
that is more commonly seen on bridge deck construction. This machine
rides on forms or steel pipes. It is characterized by a truss extending
across the forms with a suspended longitudinal screed roller that
moves transversely across the pavement. An auger mounted on the
front end of the roller screed spreads the concrete, and the roller
screed smooths the surface.

Neither the multiple roller-type paver or the bridge-type
machine enjoy a reputation for producing a very smooth riding surface
and are unlikely to be seen on a pavement surface where a ride specification
with bonus/penalty is in effect. Productivity is also relatively
low for both types of machines.

Fixed-form paving is a form of concrete pavement construction
where fixed forms are used to hold the concrete in place at the
proper grade and alignment during construction. This type of paving
is different from slip-form paving, which utilizes the forms of
the slip-form paving machine to form or mold the concrete in place
at the proper grade and alignment, in lieu of fixed forms, during
paving. In general, fixed-form paving is not as productive as slip-form
paving because of the difference in the efficiency of the placing,
spreading, and finishing operations with form riding equipment.
In addition, it takes time and effort to set and remove forms before
and after paving. However, fixed-form paving is more applicable
than slip-form paving in certain situations, such as ramps, blockouts,
small paving areas, or where slip-form paving is not feasible or
economical. Most of the main lane concrete paving in TxDOT projects
is done by slip-form paving. Usually, only minor portions of the
concrete paving are done by fixed-form paving.

To facilitate the placing, spreading, consolidation, and finishing
operations, concrete used for fixed-form paving usually has more
workability or higher slump than the concrete used with slip-form
paving.

6.2.1 Forms

A key element in fixed-form paving for constructing a smooth
concrete surface is the form. Typical forms have the following characteristics:

Flange braces extend outward from the
base not less than 2/3 the depth of the form.

The final grade and smoothness of the concrete surface is
determined, to a large extent, by how secure and how close the forms
are set to the final grade and alignment lines. One of the most important
requirements of the forms is to provide stability while concrete
is being placed. Unstable forms will cause irregularities in the
finished concrete pavement. The top and the face of the forms should
be as flat as possible. A straightedge or stringline can be used
to check for variance. Forms must be free from detrimental kinks,
bends, or warps that could affect ride quality or alignment. The
forms should allow for tightly locking the ends of adjacent form
sections. The ends of the forms should be flush when they are in
position. All forms must be cleaned and oiled before use.

6.2.2 Setting Forms

A survey line is established to facilitate setting forms at
proper grade and alignment. Form setting is a critical construction
operation since the final grade and smoothness of the pavement surface depends,
to a large extent, on how accurately the forms are set to line and
grade and how well and uniformly the forms are supported by a firm
foundation.

The finished smoothness of the pavement depends on the care
with which the forms are set and maintained because the finishing
equipment rides on the forms. Proper alignment and elevation of the
forms will contribute to a smooth pavement. It is important to provide
a firm and level foundation under all forms. Figure 9-21, Figure 9-22,
Figure 9-23, and Figure 9-24 illustrate the process of form setting.

Once the forms have been set, they are checked for overall
alignment and tolerance before any paving takes place. If any form
section is out of line, it needs to be corrected immediately. Joints between
forms must be tight and smooth. Specification Item 360 requires
that the contractor provide metal side forms of sufficient cross-section,
strength, and rigidity to support the paving equipment and resist
the impact and vibration of the operation without visible springing
or settlement.

The preferred depth of the form shall be equal to the required
edge thickness of the pavement. Forms with depth greater or less
than the required edge thickness of the pavement can be used if
the difference between the form depth and the design pavement depth
is not greater than 2 in., and:

forms of a depth less than the pavement
edge thickness shall be brought to the required edge thickness by
securely attaching metal strips or wood shims of approved section
to the full width and length of the base of the form, as shown in
Figure 9-25. Use grout to fill the gap with wood shims to establish
grade.

Unlike a slip-form paving operation, where the thickness of
concrete pavement can be adjusted for “knots” or rises in the base
to maintain proper thickness, the thickness of concrete pavement
is established by the height of the side forms. That is, side forms
that are 8 in. tall will produce a concrete pavement that is at
least 8 in. tall only if the base between the two forms does not
have an area that is above the level of the bottom of the forms.
To check that there are no bumps in the base, a “scratch” template
may be used.

The scratch template is typically a lightweight truss that
is supported on the side forms by wheels and has long metal tines
spaced about a foot apart that extend down from the truss for the
thickness of the concrete pavement. The scratch template is moved
along the side forms. If the tines on the scratch template come
in contact with the base and “scratch” the surface, that would indicate
an area with a rise in the base and where the concrete pavement
would have insufficient thickness. If there is a high spot in the
base, then that area needs to be milled down, or more commonly,
the side forms need to be shimmed up to achieve proper concrete
pavement thickness. Base planers were used in the past with side
form paving to excavate excess base material. These planers were
suitable for use on non-stabilized base materials. Any milling of
stabilized base should be approved by the engineer to ensure that
no damage to the base or deficient base thickness result.

6.2.4 Paving Operations

As with any paving method, it is important that the concrete
be discharged, consolidated, and finished to provide optimum ride
quality and long-term performance. Equipment used for placing and finishing
concrete in fixed form paving varies substantially from project
to project, and detailed descriptions of the placing and finishing
operations are not provided in this document.

One of the key elements in fixed-form paving is to maintain
a consistent and uniform head of concrete in front of the strike-off
screed. The strike-off screed used in fixed-form paving is lighter
than the one used in slip-form paving, and too much variation in
the head of concrete will result in reduced smoothness of the concrete
surface. A head of concrete that does not run over forms or the screed
works best. Also, steady machine progress improves pavement smoothness,
and concrete should be delivered to ensure steady machine progress.

Finishing operations such as floating, burlap/carpet drag,
tining, and curing operations are similar to those in slip-form
paving, and they are described under “Slip-form Paving.”

Figure 9-26, Figure 9-27, and Figure 9-28 show how a vibrating
screed is pulled forward with a winch at each end of the screed
connected to a cable hooked ahead of the screed. A manually operated
finishing screed shall be a strike template and a tamping template
or a vibratory screed with adequate length to cover the width of
the slab.

Figure 9-28. Knock down the ridge of mortar formed by
the Clary screed.

When the concrete is setting up on the screed and the screed
needs to be cleaned, have the contractor move the screed ahead of
the concrete so that cleaning water does not fall on the plastic concrete.
Figure 9-29 shows the cleaning of the screed between haul trucks.

Forms can be removed as early as practical without damaging
the concrete. In most instances, forms can be removed within 6 to
8 hr. after concrete placement. The forms shall be carefully removed
in such a manner that minimal or no damage will be done to the edge
of the pavement. All damage resulting from this operation and any
honeycombed areas shall be repaired with a mortar mix within 24
hr. after form removal. Immediately after removing the side forms
from the concrete pavement, apply membrane cure to all concrete
surfaces not previously treated. Forms should be cleaned right after
removal. If not, they become difficult to clean. Figure 9-30 shows
removed side forms.

6.3 Slip-form Paving

Slip-form paving is the most widely used paving method in
modern concrete paving construction. The slip-form paver consolidates,
screeds, and initially finishes the concrete in one continuous operation
without the need of forms. Modern slip-form paving equipment is
of the extrusion-type process. The extrusion process can be simply
defined as forcing, pressing, or pushing a material through a die
or mold to create the desired shape. Slip-form paving is very efficient
and can provide smooth concrete pavement. Because there are no forms,
the plastic concrete must be able to hold the pavement edge. When
slip-forming, the desired slump of the concrete is 1-1/2 in.

6.3.1 Alignment and Grade

The “stringline” is actually a slender wire rope. It is usually
tensioned fairly high to reduce sags in the wire between the supports.
A sensor on the paving machine will follow the stringline and any sags
will show up in the final ride surface as waves that can produce
an unpleasant ride quality.

A stringline is established to control the slip-form paving
equipment at the proper grade and alignment. A surveying crew establishes
a stake line every 25 or 50 ft. along, but offset from the edge
of the pavement to be placed. Wood stakes used for this line are
about 1-1/2 to 2 in. square, commonly called hubs, and are driven
into the ground. The alignment is then established with a tack in
the top of each stake. Grade is also established by elevation grading
of each stake. For hard, dense subgrade such as an asphalt bond
breaker, nails driven into the subgrade are used instead of wooden stakes
with tacks. Using the line and grade from this survey line, a stringline
is established for the slip-form paving machine to follow utilizing
the machine’s electronic grade and alignment controls. Usually,
another graded stake line is established along, but offset from,
the other edge of the pavement to control the grade of that side
of the paving machine; however, alignment is only controlled from
one stringline.

6.3.2 Overview of Slip-form Paver

Slip-form paving is accomplished by the use of several self-propelled
machines in a line which is commonly known as a paving train.

The first machine in line is a concrete placing machine and,
depending on the manufacturer, this machine is sometimes referred
to as a placer/spreader. This machine receives the mixed concrete from
the delivery vehicles and places and spreads the concrete in front
of the second machine, which is the slip-form paver. Sometimes this
first machine is eliminated if the concrete can be deposited directly
in front of the slip-form paver from the concrete delivery units
or if a concrete placing attachment is installed in the front of
the slip-form paver.

The slip-form paver spreads the concrete uniformly across
the paving area with an auger, consolidates the concrete with spud
vibrators, and strikes off the top of the concrete to a suitable
elevation to feed into the mold that shapes the pavement into the
proper geometric configuration. Depending on the manufacturer, some
slip-form pavers also utilize what is known as a tamping bar. The
tamping bar slightly tamps large aggregates into the top of the
concrete slab to prevent the paver's mold from snagging the aggregate
and causing a tear in the top of the slab.

The third machine in the train is a tube float. This machine
smooths and seals the top of the pavement by dragging diagonally
mounted aluminum tubes forward and back along the top of the pavement.
This machine is sometimes eliminated by attaching what is known
as an auto float to the back of the slip-form paver. The auto float
automatically travels across the top of the pavement while simultaneously
oscillating in a forward and back motion to smooth and seal the
top of the pavement.

The last machine in the train is a combination tine/cure machine.
This machine installs the tining in the pavement top with a metal
comb that is automatically dragged across the top of the pavement. This
machine is then used to spray the curing compound on the pavement.
Sometimes a second curing machine is required if the tine/curing
machine can’t perform the curing operation in a timely manner. Either
the tube float or the tine/cure machine is also used to install
any required texture, such as a burlap or carpet drag texture, after
all finishing is completed and prior to any tining texture required.

Slip-form pavers contain various combinations of all or some
of the following components: auger spreader, spud vibrators, oscillating
screeds, clary screed, tamping bars, and pan floats. A slip-form paver
is shown in Figure 9-31.

Slip-form pavers are equipped to spread the concrete uniformly
and strike off the concrete to the required section, using a power
driven device, either a reciprocating blade, a screw conveyor (auger),
or a belt conveyor, without loss of traction. A slip-form paver
with two augers is shown in Figure 9-32.

A machine with two tracks is steered by varying the speed
of the tracks from one side to the other side. The bigger machines
with four tracks, two tracks on each side, are stirred by pivoting
each track, much like the front wheels on a car. The tracks can
either ride on the base or on a previously placed pavement. Figure 9-33
shows a paver machine on tracks.

The base should extend out, past the paver’s tracks, to give
adequate support for the machine. This is illustrated in Figure 9-34.
The track path should be level and clean to allow for a smooth concrete surface.
Do not force the grade control sensor to make up for trash, such
as spilled concrete or tie bars, in the track path.

Usually, slip-form pavers have an electronic sensor system
or equivalent to provide grade control. Figure 9-35 and Figure 9-36
show examples of such sensors. Sensors come in both electronic and hydraulic
models. The electronic sensors work better.

The grade control sensors, one on each side of the paver,
have wands that ride on the bottom of a guide wire that adjusts
the height of the entire machine by raising or lowering the vertical
hydraulic cylinders on each side of the machine. Figure 9-37 shows
vertical and horizontal sensors. The guide wire is supported and
tensioned to prevent any measurable sag. Figure 9-38 shows the side
view of an idle paver. Notice the two hydraulic cylinders on the
side of the paver (right side of Figure 9-38). Figures 9-39 and
9-40 show the guide wire controls.

The alignment or steering of the paver can be either sensor-controlled
or operator-steered.

6.3.5 String-less Concrete Paving

The following information is taken from the FHWA Concrete
Pavement Road MAP. Conventional concrete paving with a slip-form
paver requires the installation of a stringline and support posts adjacent
to the roadway to establish the correct pavement alignment and profile.
The stringline adds several additional feet (+/- 6 ft.) of required
clearance to the paving envelope, which is already wider than the
pavement due to the tracks of the slip-form paver. In addition,
the stringline becomes an obstacle for equipment, concrete delivery
trucks, and finishing crews. If equipment access across the stringline
is required, the stringline must be lowered and reset, resulting
in delays and introducing the potential for errors.

String-less paving is a technology that eliminates the installation
and maintenance of stringlines and has the potential to decrease
the need for surveying and increase the smoothness of the pavement
profile. The benefits that can result from string-less paving include
increased production, decreased construction time, and reduced potential
for errors.

Several companies have developed string-less equipment control
and guidance systems using technologies such as global positioning
systems (GPS), robotic total stations, and laser positioning. String-less
technology replaces the traditional stringlines with an electronic
tracking process that controls the horizontal and vertical operation
of the slip-form paver. The construction industry has been using
string-less technology for elevation and steering control of equipment
for a number of years. To date, the extensive use of this technology
has been applied to grading operations. However, string-less paving
is an emerging technology for concrete paving because it has the
potential to allow contractors and owner/agencies to receive production
benefits (e.g., reduced survey costs, fewer construction hours)
while still meeting smoothness requirements. Figure 9-41 shows the string-less
paving machine.

Contractors using string-less paving equipment are still required
to establish control points at maximum intervals of 500 ft. and
use the control points as reference.

6.3.6 Paver’s Forward Speed

The speed the paver moves forward is controlled by the operator.
The speed should be as uniform as possible, but should vary with
the rate of concrete delivery so that complete stops are held to
a minimum. The paver speed is up to 20 ft. per minute.

If the paver stops moving forward, the vibrators must be turned
off within 5 seconds. Continued running of the vibrators will result
in segregation of the concrete, forcing water and fines to the surface.

6.3.7 Augers

Rotating the augers moves the concrete sideways across the
entire lane being paved. The augers are controlled by the operator
and can be turned either clockwise or counterclockwise as needed.
Figure 9-42 shows a close-up of an auger.

The augers on the spreader of a paving machine help distribute
the concrete over the width of the pavement placement ensuring that
a uniform head of concrete is maintained across the pavement width
and extending well above the vibrators. This overburden over the
vibrators helps direct the energy from the vibrators downward into
the concrete pavement and helps ensure proper consolidation of the
concrete. Figure 9-43 shows the spreading of the concrete.

Vibrators need to be mounted high enough that they will not
snag any of the reinforcing steel. An isolated streak in the concrete
pavement behind the paving machine may indicate that there is a non-working
vibrator.

Figure 9-46 shows the normally recommended vibration position.
The paver is moving to the left. A concrete head or surcharge should
be maintained over the vibrators during placing operations. The
vibrators must be turned off whenever the forward motion of the
paver is stopped. Figure 9-47 shows the vibrated zones.

No vibrator streaks or trails should be apparent behind the
paver. Figure 9-48 shows unacceptable vibrator streaks. Here, the
vibrators are not vibrating enough. Because of this, the vibrators
are plowing a trail instead of vibrating the concrete.

After screeding, the concrete surface is finished with a pan
float that further smooths and consolidates the concrete. Pan floats
are solid plates anywhere from 18 to 60 in. wide and slightly narrower than
the pavement width. Figure 9-49 shows pan floats on an idle paver.

6.4 Placing Concrete

As mentioned above, concrete can be placed in front of the
paving operations using a number of methods. Regardless of which
method is used, concrete should be placed as near as possible to
the final location. Concrete should never be moved using rakes or
vibrators, as these methods cause segregation. Concrete should only
be moved by the use of shovels or the augers mounted on the spreader
and paver.

In order to promote proper hydration, concrete placed should
have a temperature between 40°F and 95°F. It is allowed to use the
loads that are already in transit when the temperature was found
to exceed the allowable limits. Immediate action must be taken or
paving operations ceased until the concrete temperature is corrected.

When the ambient temperature in the shade is 40°F and falling,
concrete should not be placed. Typical concrete paving mix designs
have relatively low cement contents and usually contain fly ash. This
combination of materials tends to make the concrete susceptible
to prolonged set times and slow strength gains in cold weather conditions,
which can lead to plastic shrinkage cracking. If the ambient temperature
is expected to drop below freezing, precautions must be taken to
ensure the freshly placed concrete is protected against freezing.

Placing concrete during hot weather can also lead to plastic
shrinkage cracking if proper curing is not performed as soon as
possible after texturing operations. If there is a delay in application
of the curing method, measures should be taken to ensure that the
unprotected pavement surface is kept from drying. The use of fog
sprays, wind screens, or evaporation retardants are acceptable methods to
prevent the surface from drying.

6.5 Finishing Operations

To close all surface openings and produce a uniformly smooth
and flat surface, the plastic concrete behind the screed is either
floated or straightedged, or both.

6.5.2 Floats

A float may be used only when using a finishing machine. Figure 9-50
shows an example of tube floats. Tube floats are pulled back and
forth in the longitudinal direction. Notice that each tube float is
suspended and pulled by chains. The tube floats are set at 60 degree
angles to the pavement’s centerline. The leading end of the tube
float may be on either the inside or outside of the pavement. The
tube floats must extend across the entire width of the pavement
being placed. For wide concrete placements, more than one tube is
used.

Tube float machines must have spray nozzles to provide a fine,
light fog mist to the concrete surface just ahead of the tubes.

A longitudinal float, also called auto float, slides back
and forth in the transverse direction. An example of a longitudinal
float is shown in Figure 9-51. Notice the surface marks left by
the float.

Another type of float is the drag float, an example of which
is shown in Figure 9-52. Because a drag float is heavier than a
tube float or a longitudinal float, the drag float applies greater
surface pressure, making it more effective at smoothing out a knot
or bump in the plastic concrete. A drag float needs to be more effective
because it only makes a single pass across the plastic concrete.

Care must be exercised while floating to avoid distorting
the surface. A bull float (Figure 9-54) can produce a depression
if excessively used in one spot. Longer bull floats will produce
better results. Bull floating should be limited to small areas and
should not be used to float the entire surface. Avoid using a bull
float, if possible.

A straightedge is a 10-ft. or 15-ft. long, steel or magnesium,
square tube, which contacts the concrete surface. An illustration
of straightedge finishing is shown in Figure 9-55 and Figure 9-56.
A straightedge flattens the concrete surface and in the case where
floats were not previously used, can close surface tears produced
by a screed. A straightedge is better at flattening surface bumps
and seams than a float, but using a straightedge takes longer than
floating because it cannot be mechanized. The straightedges should
be parallel to the centerline of the pavement.

Notice in Figure 9-56 that the surface no longer shows the
irregularities left by the longitudinal float.

6.5.4 Bleed Water and Free Surface Water

Finishing should be held to a minimum during the period of
greatest bleeding since working the surface in the presence of excess
water leaches out a portion of the cement and produces low-strength
surface mortar. The use of supplementary cementitious materials,
such as fly ash and slag cement, reduces bleeding water.

Under calm wind and high relative humidity conditions, the
concrete pavement should provide enough bleed water that little
or no water needs to be added to float and finish the concrete surface. Under
very dry conditions or when there is a strong breeze, the bleed
water may evaporate rapidly before floating can be performed. A
fine mist of water may be used to reduce the loss from evaporation,
but never to form slurry. The fine mist should come from a misting
nozzle mounted on a pipe frame that is not pointed directly at the
pavement surface.

Water is not allowed to be applied from a nozzle or thumb
over a garden-type hose as shown in Figure 9-57. Adding water to
the surface makes the finishing easy to perform. However, this practice dilutes
and washes away the surface mortar, resulting in poor performance
of concrete pavement in terms of skid resistance, scaling, and durability.

If excessive mortar slurry is present during finishing operations
(see Figure 9-58) without the addition of water, this generally
indicates an issue with the concrete mix. Batch plant operations
should be reviewed to correct this situation.

Finishing operations should not be conducted when free water
is on the surface of the concrete. Finishing with water on the surface
will cause the cement at the surface of the concrete to become diluted
or even washed away as shown here. Wait until the water evaporates
before continuing with the finishing.

Once the finishing operation is completed, measures should
be taken to keep the surface from drying before the curing compound
is applied. It may take a while for the concrete to set up sufficiently after
finishing before the texturing can be performed. This delay may
increase if the concrete temperature is cool or a high dosage of
a set-retarding admixture is used in the concrete mix.

One method of keeping the surface from drying is the use of
an evaporation retardant. The evaporation retardant is applied in
a fine mist to uniformly cover the pavement surface. An important property,
in addition to reducing evaporation, is that it is specifically
formulated to not harm the concrete when final texturing of the
surface occurs.

The evaporation retardant (see "DMS-4650, Hydraulic Cement
Concrete Curing and Evaporation Retardants") should be applied according
to the manufacturer’s recommendation when finishing operations cease
and maintained until the final texturing occurs. This may be before
or after the carpet drag is performed.

Historically, TxDOT has required the contractor to have a
small quantity of evaporation retardant (typically in a small garden-type
sprayer) on the project to be used if there was a temporary breakdown
of the paving machine or a brief disruption in the delivery of concrete.
In these circumstances, the contractor would apply the evaporation
retardant to the mass of concrete in front of the paving machine
to reduce evaporation and keep a crust from forming on the concrete
until paving could resume.

6.6 Texturing Operations

Surface texture is required to provide skid resistance and
to prevent hydroplaning. Skid resistance is achieved from a combination
of the fine aggregate and carpet drag. Hydroplaning is minimized by
tining. A carpet drag texture and a metal-tine texture finish are
required for all areas with a posted speed limit in excess of 45
mph. When shown on the plans, a carpet drag texture can be the only
surface texture required for areas with a posted speed limit less
than 45 mph.

When carpet drag is the only surface texture required by the
plans, ensure that adequate and consistent coarse texture is achieved
by applying sufficient weight to the carpet and keeping the carpet from
getting plugged with grout. The target carpet drag texture is 0.04
in., as measured by Tex 436-A. Any location with a texture less
than 0.03 in. must be corrected by diamond grinding or shot blasting.
The engineer will determine the test locations at points located
transversely to the direction of traffic in the outside wheel path.

Figure 9-59 shows an upside-down artificial carpet pulled
longitudinally on the plastic concrete. This gives the concrete
surface a rough texture similar to a broom finish.

The carpet shall be a single piece long enough to reach across
the width of the plastic concrete and having enough longitudinal
length in contact with the concrete to produce the desired texture.
The engineer may allow changes in the length and width of the carpet
to accommodate specific applications.

To keep the mortar from hardening on the carpet, the carpet
should first be wetted before using and it should be cleaned with
water often enough to produce a uniform finish on the concrete.
Streaking of the surface behind the carpet is caused by hardened
grout on the carpet. The contractor should stop the drag as soon
as the streaking is observed and remove any dried grout from the
carpet.

Figure 9-60 shows the mounting of a carpet. The carpet shall
be mounted on a movable support system capable of varying the area
of carpet in contact with the pavement and can be pulled either mechanically
or manually.

To keep the carpet in contact with the concrete, the trailing
end of the carpet may be weighted down with something that will
provide enough weight to produce the desired texture.

Keep an eye on the drag ‑ it can indicate some things about
the concrete. If it starts to catch and pull aggregates to the surface
instead of leaving a uniform texture, there are two things that
could be wrong:

There may not be enough sand in the mix
to allow grout to be worked to the surface.

After the carpet drag operation is completed and the concrete
surface has set enough, tining operations can begin. The default
tining is transverse (see Figure 9-61); however, longitudinal tining
is acceptable when shown on the plans. The metal tines should make
grooves in the plastic concrete which are 3/16 in. deep and 1/12
in. wide. The minimum groove depth is 1/8 in. The grooves shall be
spaced approximately1 in., center-to-center, for transverse tining,
or 3/4 in., center-to-center, for longitudinal tining. If noise
is a concern, longitudinal tining has been shown to be quieter than transverse
tining.

To ensure that the grooves are straight, the tines should
be pulled by a mechanized device. The mechanical device is designed
to tine up to a 24-ft. wide concrete pour. Figure 9-62 shows a mechanical
tining device.

6.7 Curing the Concrete Pavement

Curing is the process by which hydraulic cement concrete matures
and develops hardened properties over time as a result of the continued
hydration of the cement in the presence of sufficient water and
heat. In concrete pavement, the surface area is relatively large
compared with the volume. Moisture can be evaporated from this large
surface area unless measures are taken to keep the moisture in the
concrete. With loss of moisture, the surface concrete will not develop
enough strength and could result in scaling, spalling, and other
distresses. It is important that proper curing is provided. The
most widely used curing method is the use of curing compound.

6.7.1 Curing Compound

Curing compound should meet the requirements of Type 2 membrane
curing compound in "DMS-4650, Hydraulic Cement Concrete Curing and
Evaporation Retardants." Curing compound is similar to paint and
must be thoroughly agitated just before using. The curing compound
should not be thinned or diluted. The curing compound should be
re-agitated during its use if it becomes non-uniform.

Two separate coats of curing compound are to be applied to
all exposed concrete surfaces. Applying the membrane cure in two
coats has several advantages over a single coat:

it helps ensure a more even coating of
the concrete surface if one or more of the spray nozzles were partially
obstructed.

The first and second coats should be applied within 10 min.
and 30 min. after completing texturing operations, respectively.
The individual application rate should be not more than 180 ft.2/gal.

Where the coatings show discontinuities, pinholes, or other
defects, or if rain damages the newly applied curing compound, an
additional coat of the compound shall be applied.

Once dried, the curing compound will bead water. If the water
does not bead but instead wets the surface of the concrete as determined
by darkening the concrete, or by visibly soaking into the surface,
an additional coat or coats of the curing compound should be applied
immediately.

Should the membrane be damaged from any cause before the expiration
of 72 hr., the damaged portions should be repaired immediately with
additional compound.

A mechanized spraying unit (see Figure 9-63) is essential
when more than one lane of pavement is placed. Note that the machine
is applying curing compound to the vertical edge of the concrete
as well.

Figure 9-67 shows that the tie bars have been covered with
a plastic sleeve to prevent curing compound from dropping on the
tie bars. Any curing compound that ends up on the tie bars or on
the “U’s” for the curbs will need to be removed with a steel brush
or by sand blasting to ensure proper bonding of the steel to the
yet-to-be-placed concrete.